Dental investment material



Patented Nov. 26, 1940 UNITED STATES DENTAL INVESTMENT MATERIAL ThomasE. Moore, Toledo,

Ohio, assignor to The Ransom &,B.andolph Company, Toledo, Ohio,

a corporation of Ohio No Drawing. Application March 29, 1937, Serial No.133,654

5 Claims.

This invention relates to a dental investment material and moreparticularly a material suitable for casting high melting alloys.

The object of the invention is to provide a 5 dental investment materialwhich will combine with the other necessary characteristics thepossibility of being employed to cast high melting alloys withoutcausing imperfections therein or corrosion thereon.

The most commonly used investment material for making dental casting ofgold and similar alloys is a mixture composed chiefly of calciumsulphate hemi-hydrate and silica with various minor ingredients forespecial purposes. 15 A widely used composition of this kind is made inaccordance with my Patent No. 1,924,874, for Method of making dentalcastings and composition employed in said method, which issued August29, 1933.

Investment materials of this type are very satisfactory for ordinarygold alloys, but with gold alloys containing platinum or iridium so thatthey melt at 2300 F. or higher and for al- 25 loys of chromium, nickel,cobalt and beryllium,

with similar high melting points, investment material containing calciumsulphate hemi-hydrate has not been fully satisfactory. When such hightemperatures are reached, SO: and S02 are released in such quantities asto have a troublesome corrosive effect upon the metal, These gases arereleased to some extent at 1800 F., but the difficulty becomes much moremarked at temperatures above 2000 F., and has pre-' 35 vented the makingof satisfactory castings in this type of mold at 2300" F., or above. Theresult has been that high melting alloys of the type mentioned above,when cast in molds made of standard materials including calcium sulphatehemi-hydrate, would have imperfections, especially roughened surfaces.

By my present invention, the commonly used investment materialcontaining calcium sulphate hemi-hydrate, such as disclosed in my abovecited patent, can be employed for casting these high melting alloyswithout producing the imperfections due to corrosion. This is effectedby including in the investment material compounds which will releaseneutral gases during the heating of the investment material and in thisway prevent any released sulphur compounds from producing corrosiveeffects. While various materials which introduce inert or protectivegases may be employed, substances rea leasing carbon dioxide have beenparticularly available and useful. Consequently, the use of thesematerials will be described more fully.

It has been found desirable to introduce materials which will releasecarbon dioxide during the heating up of the mold and will continue to doso when the final temperature is reached during the casting operation.The release of the carbon dioxide during the earlier heating stagesfills the mold and the pores thereof with the gas, but it is desirablethat the release of carbon dioxide shall be continued due to the heat ofthe metal in the casting operation so as to more fully protect thecasting from the action of the sulphur compounds. The continuous releaseof the carbon dioxide is most easily effected by employing severaldifferent compounds which will result in the release or production ofcarbon dioxide at successive stages of heat. In one very convenientcombination of such compounds, oxalic acid or a salt thereof, such ascalcium oxalate for example, may be used to produce carbon dioxide atrelatively low temperatures. For the next stage, one or more salts ofcarbonic acid such as calcium carbonate may be employed, which willrelease carbon dioxide during the intermediate stages of heating. Forthe production of carbon dioxide during the final stages of heating,carbon may be employed in any convenient form. Lamp black, powderedcharcoal, colloidal graphite and flake graphite have been used withsuccess. The free carbon continues to unite with oxygen at castingtemperatures as high as 2300 F. It will be readily understood that thecompounds cited are mentioned by way of example and other equivalentcompounds may be employed, although those noted are readily availableand in practice have given good results. The inclusion of 1% each ofcalcium oxalate, calcium carbonate and graphite in an investmentmaterial such as described in my above cited patent has been foundsuflicient to prevent the corrosive effects of the sulphur compoundswhen casting alloys such as mentioned having a melting point of 2300 F.or higher.

As is well known, oxalic acid decomposes so as to release carbon dioxideto a material extent at 320 F. and while some of the oxalates are moreresistant they begin to decompose well below 1000 F. Calcium oxalate,for example, decomposes to a material extent at a temperature of 600 F.Therefore this invention provides an investment material which begins torelease carbon dioxide at a temperature below 55 800' l". and continuesto add carbon dioxide up to a temperature above 2000' 1".

What I claim is: 4 1. A mold material for making castings at atemperature as high as 2000' F. comprising calcium sulphate hemi-hydrateand materials that begin to release inert gas at a temperature below1000 F. and will continue to release inert gas at a temperature as highas 2000' F.

10 2. A mold material for making castings at a temperature as high as2000 F. comprising calcium sulphate hemi-hydrate and materials thatbegin to release carbon dioxide at a temperature below 1000 F. andcontinue to release 16 carbon dioxide at a temperature as high as 3. Adental investment material comprising calcium sulfate heml-hydrate, anda combination 01' materials that begin to release carbon diox- 20 ide ata temperature as low as 600' F. and con-'- tinue to releasecarbondioxide while the materialisbeingheatedtoatemperatureashigh as 1800' FL,said, materials including at least three ingredients which begin torelease carbon dioxide at diirerent temperatures. 5 4. A dentalinvestment material comprising calcium sulphate hemi-hydrate. an oxalateand calcium carbonate, said oxalate being present in a kind andquantity'that will begin to give 01! carbon dioxide at a temperatureaslow as 600 F. and will continue to give ofl carbon dioxide during thenormal heating of the mold until the calcium carbonate starts to giveofl carbon dioxide. 18 5. A mold material for making castings at atemperature as high as 2300' '1'. comprising sulphate heml-hydrate,silica, an oxalate, a carbonate and graphite.

THOMAS E. MOORE.

